The design of nails for use in a nail strip must take into account a variety of factors.
1. The nail itself must have adequate holding power for the purpose intended, this factor usually being determined by the thickness of the nail shank.
2. The nail head must have adequate holding power to withstand driving forces developed by the nailing piston, i.e. it must not drive into the work piece too far.
3. The nail head must have sufficient strength to secure the work piece into which the nail is driven.
4. The nail head should be of such a design that it does not interfere with the shanks of adjacent nails in the strip. Any interference between the nail head of one nail and the shank in the next nail, will result in increased spaces between the nails in the strip and cause angular misalignment. Angular misalignment will cause either jamming of the nailing tool or misfiring. The extra space reduces the number of nails in the strip. This in turn will increase the frequency with which the applicator must be temporarily taken out of service while the nailing strip is replenished.
5. The nail head must be of such a design that it facilities the assembly and packaging of the nails into the strips, without the requirement of special handling.
6. The nail head must be such that it is capable of being formed at high speed on relatively speaking conventional machines, so as to avoid undue costs.
7. The nail head must be securely formed on the shank so that it does not become detached during use.
A large number of different nail designs have been proposed in the past, which are designed to meet some of these objectives. Examples are shown in Sygnator U.S. Pat. No. 5,056,976, Lingle U.S. Pat. No. 3,152,334, Sillars U.S. Pat. No. 3,031,670, and Juilfs U.S. Pat. No. 2,940,081.
It is not necessary to analyze each of these earlier patents. It is sufficient to say that while individually they may have met one or more of the foregoing objectives, individually none of them have met all of these objectives, and, even when considered collectively, none of them would be capable of meeting all of these objectives.
Most of these proposals have been based on a nail having a nail head having an effective centre which is offset to one side of the central axis of the nail shank. In particular, such nail heads have a portion which is non-circular, and is either formed with a notch, or a portion of the head is simply clipped off. The forming of a notch in the nail head results in certain problems in assembling the nails into a strip. In addition, it is difficult to form such notched nail heads with repeatable accuracy bearing in mind the type of tooling and the speeds of operation.
A nail having a head portion which is simply clipped off, while be readily manufactured and assembled, presents problems in that the holding power of the nail head is reduced.
In addition, any asymmetric shaped head can easily be cracked or snapped off while it is driven into the workpiece.
An improved form of nail head meeting most of these objectives is shown in U.S. patent application Ser. No. 08/202,016, inventors Antonio Cario, Vittorio Iannucci and Eberhard F. Leistner, filed Feb. 25, 1994, and owned by the same owner as the present application.